Helmets are often worn in sports or other physical activities to protect from injuries that can result from impact forces and/or accelerations to the brain. Helmets can be generally classified into two categories using different impact attenuation technology: single impact helmets and multiple impact helmets. Design constraints for any helmet typically include overall size, weight, aesthetic commercial ability of the concept, and compliance with all appropriate governing impact standards associated with the particular category of the helmet.
In single impact helmets such as typical cycling, alpine and motorcycle helmets, the shock absorbing elements usually undergo permanent deformation under impact. In multiple impact helmets such as typical hockey, lacrosse, and football helmets, the shock absorbing elements are designed to withstand multiple impacts with little to no permanent deformation.
Some multiple impact helmets use either vinyl nitrile (VN) or expanded polypropylene (EPP) material. These materials can exhibit performance degradation after multiple impacts due to slight plastic deformation after each impact, which may cause a reduction in the material thickness in the impact zone thus an increase in material density, which makes the material harder and may result in reduced energy management.
Other known multiple impact helmets include a shock absorbing layer of compressible cells containing a fluid, for example air, the cells being closed except for a small passageway allowing the fluid to escape when the cell is compressed. The structure of the cell is typically such as to resist compression at the initial phase of the impact, the passageway having a choking effect on the fluid moving at high velocity; the cell then progressively compresses as the fluid is slowly vented out through the passageway. Such a mechanism however requires the individual cells to have a relatively large size, in order for the volume of fluid contained therewithin to have an effect on the cell's resistance to impact. The use of larger cells may prevent optimized coverage of the shock absorbing layer within the helmet, thus hindering achievement of proper all around protection
Due to insufficient measuring techniques at the time, it was commonly viewed in previous research that linear and angular accelerations strongly correlated with respect to head injury criteria during impacts; this lead scientists to only focus on linear accelerations to determine head injury thresholds, as it was the easier of the two accelerations to measure. As such, helmet standards to date currently only measure linear accelerations as their pass/fail criteria, with no mention of angular accelerations.
New research evidence seems to indicate that angular accelerations can vary significantly from linear accelerations under certain impact conditions, and potentially can even solicit greater forces therefore causing more damage and injury if not managed appropriately. For example, angular accelerations can be significant and even predominant when an impact is received off of the center of mass thus causing a greater degree of rotation, a scenario which is very likely to occur in all sporting activities where a helmet is needed for protection.
Generally speaking, as the density, stiffness, and thickness or height of the shock absorbing elements are varied, proportional linear impact management characteristics are obtained. However, typical known shock absorbing elements provide little angular acceleration impact attenuation.
For example, one type of known impact technology uses a plurality of shock absorbing members interconnected with webbing. The webbing typically allows for loads to be transmitted between the members, thus restricting lateral displacement during collapse of the interconnected members. The webbing also increases the resistance to bending of the tubular members, and as such may prevent adequate angular acceleration impact attenuation.
Accordingly, improvements are desirable.